A synchronous machine is an electric machine which can be operated as either a synchronous motor (synchronous motor mode) or a synchronous generator (synchronous generator mode). Conventionally, a synchronous machine has two separate and independent exciter field windings. Also, conventionally, two separate and independent control units have been used, one control unit for the exciter field winding for the synchronous motor mode and another control unit for the exciter field winding for the synchronous generator mode. The use of two exciter field windings and two control units make the synchronous machine and the system in which it is being used more complicated, heavier, and less reliable. The dual excitation components of a conventional synchronous machine may represent 20 to 30% of the total volume and weight of the synchronous machine. Some conventional systems use only a single, reconfigurable field winding, but still use two separate and independent control units, which then use switches or contactors to connect the appropriate control unit to the field winding. Dual field windings, dual control units, and/or switches and/or contactors add cost, weight, volume, and complexity to the system, and adversely affect the overall reliability of the system. U.S. Pat. No. 5,770,909 to Rosen et al., hereby incorporated in its entirety herein by reference, discloses a synchronous motor-generator system which uses a rotary transformer.
Conventional synchronous machines also use a low frequency excitation current and large field windings are used to avoid energy losses. These large field windings substantially increase the amount and weight of the expensive copper used in the windings. Further, with the conventional low frequency excitation current, the back electromotive force generated in the field windings is significantly affected by the rotor speed, and this can cause stability problems during the startup process.